Determination of the rubber content of latex



De@ 17, 1935 J. s. WARD Erm. 2,024,617

DETERMINATION 0F THE RUBER CONTENT 0F LATE-x Filed Dec. 2l, l1932 3 Sheets-Sheet l 'iL-h 53j5a 3mm ($292 [/0/777 S. Ward and 32 an'vue/ D. Ge/vmcm 55 e 57 If 50 5 38 a" i 53 T Dec. '17, 1935. J. s. WARD l-:r Al. 2,024,617

DETERMINATION 0F THE RUBBER CONTENT OF LATEX Filed Dec. 21, 1932 3 Sheets-Sheet 2 N wf'- FIS gmc/rm fx. 6 A 6. WCL/d anal E Samue/ D. Geman Dec. 17, 1935. J. s'. WARD E'r AL DETERMINATION OF THE RUBBER CONTENT 0F LATEX Filed Dec. 2l, 1932 3 Sheets-Sheet 3 Patented Dec. 17, 1935 nErEamA'rroN or 'ma RUBBER coN'rEN'r or. LATEX f John s. ward and samuel n. Gennian, Akron.'

Ohio, asslgnors to Wingfoot Corporation, Wilmington, Del., a corporation of Delaware Application December 2l, Serial No.' 648,219

` l vClalilll` (Cl. 8814) It is frequently desirable in the rubber indus-` A try to know the rubber content of latex. This latex, from which crude rubber is obtained by coagulation, yisa white colloidal suspension ofv 5 the consistency of thick cream and consists of rubber particles of various sizes suspended ina clear, straw-colored liquid. The actual 'distribution of the particle sizes, ranging usually fron'i 0.5 to 2.0 microns, differs somewhatasbetween 10 samples from different sources.

By knowing the rubber content of a sample of' latex, its proper coagulation and uniformity in the properties of the resulting crude rubber are greatly facilitated. Since considerable latex is l purchased b y the cruderubber producersv from various plantations, an accurate knowledge of the rubber content of the purchased latex assists in establishing a fair price for the same.v

Also, in their processes involving the use of latex, the rubber manufacturers have need. for'- accurately knowing. the rubber content of latex. .There are in general four methods, either in use or proposed, for the determination of the rubber content of latex; namely, trial coagulation, hydrometric, viscosimetric, and nephelometric.

Of these, trial coagulation is a standard pre; y

cision method which comprises'V the rapid5 coagulation of a known amountgof latex fand the subsequent creping, drying andtveighing-,of the resulting rubber. This methods lengthy and,

if shortened by omitting the final drying of the creped rubber and applying an arbitrary correction for the water content, results in loss of precision. 'f7 v The hydromtric method, using laticometers, has enjoyed rather wide adoption but laticometers have been found by many experiment- 49 ers to give unreliable rubber determination. The viscsity of either. natural or treated latex has been found to be an unreliable, criterion of the rubber content. y

Nephelometric or turbidimetric methods offer the most desirable procedures'for the determination of the rubber content of latex. The disappearance of aktest object has long been employed as a mens for comparing the turbidities of different liquids. However, ordinary turbidimeters cannot be used for the measurement of l natural or moderately `'diluted latex because of its high turbidity.

The present invention has for one of its objects to provide'` an improved method of determining the rubber content oft-latex in which a turbidimeter of simplefconstruction may be employed.

If an incandescent ilament be viewed through a turbid suspension such as latex, the filament appears yellowish and surroundedv by a field of 0- less intense dirruse light. As tpev emmer-.tpg

suspension is increased, the illamen't becomes redder and less intense with respect to the diffuse'fleld and disappears when .the two light intensities become approximately the same. The

:depth ofthesuspension for this apparent ex- 5..

tinction o f the filament depends upon the turbidity of the suspension which in turn dependsl upon the mean size and volumetric number of the suspended particles, the effective mean wave length or 'color of the light from the filament, 10.

and thef refractive indices of the two phases of the suspensic'm.-` Since'the rubber content of latex depends upon the size and volumetric number of the rubber particles, this extinction forturbid suspensions canv be used with proper precautions` to determine the rubber'content of either natural..- orconcentrated latices.

the accompanying drawings, Fig. 1 is a perspective' .view of a -suitable turbidimeter showing aconvenient form thereof; 20

Fig'. 2 is a vertical transverse sectional view taken on line II-II of Fig. l, looking in the direction of the arrow Y;

' Fig. 3 is a fragmentary vertical sectional view taken on line `III- III of Fig. 2, looking in the direction of the arrow Z;

Fig. 4 is a perspective view of one formI of extinction cell which may be used in carrying 4out the present invention;

Figs. 5 andg are" detailed elevational views 30 'of two forms of extinction cells;

' Figs. 'l to 13 show curves plotted either as extinction curves or as calibration curves which further illustrate the invention. Similar characters of reference are employed in all of the 35 hereinabove described views vto indicate corresponding parts. y

Referring more particularly to the' drawings, reference character I indicates abase upon 40 which is xedly mounted a shaft 2. -The shaft 2 supports a bracket 3 to which is fixed a lightproof cabinet "cont'aining openingsb and 6 at -its pper end and an opening i at itslower end. v

In the opening 6 placed a microscope 8 of 45 ordinaryconstuction containing adjusting screws 4vIl) and Il. The microscope 8 is supported by a bracket I3 located in an opening i4 near the upper end of the shaft 2. From a clamping means i6 through the opening I4 to another 50 opening I1 is located a splined shaft (not shown). Thus, by turning the clamp I6, the bracket I3may be suitably secured in the shaft 2.

In the opening 1 at the lower end oi? the light 55 cabinet 4,- a light bulk 2liA of ordinary construction, such as a non-focusing flashlight bulb, is detachably and adjustably located. The light bulb 20 is detachably secured to a knurled nut 22 containing a threaded screw 23 which extends ihl'mlgh a bracket 24 attached to the lower. 'ed 60 nf the iight-proof cabinet 4; screw 23 is a knurled nut 25 for the purpose of locating the bulb 20 properly in the opening 1.

'It is apparent that by suitably adjusting theknurled nut 22 laterally with respect to the openfilament of the bulb 20. The micrometer 34 is y operated by a knurled head 33. A pointer on the bracket 33 nxedly mounted on the lightproof cabinet I is provided for indicating the micrometer reading on the head 33. A scale attached to the comparator 32 is used for reading whole divisions in determining the position of the table 33 and the micrometer scale of the head 36, for reading fractional parts of the whole divisions. A pointer Il permanently mounted on the outside of the llghtproof. cabinet is provided forinon the inner rear wall of the lightproof cabinet dicatlng the reading on the scale 40. And for reading this scale when the door 9 of the lightproof cabinet is closed, a light bulb 43 supported l and arranged in parallel with the bulb 20 is provided. The switch M operates the bulb I3. When the light I3 is turned on,V the scale lll may be read by viewing it through'the opening I in the lightproof cabinet `4. When not taking a' reading, it is desirable to cover the opening 5. v Y

Mounted on the traveling table 33 is an xtinction cell 30, comprising a frame il with flanges 52 fitting in the guideways l53 of the travelingA table 33 and xed in the frame 3l is an ordinary fiat glass plate I5. the extinction cell comprises a spherical convex lens 31 mounted inthe frame 53 and hinged to the frame 5l by means of a pin 59. In place of the spherical convex lens 51 a glass biprism ITA with the apex flattened, shown in Fig. 6, may also be used. 0f course, any suitable arrangement of a glass plate for holding the sample to be tested in coniunction with a spherical convex lens or glass biprism may be used, the extinction cell herein described being used mere-v lyfor convenience in operating the instrument. It is to be understood that the term extinction cellas used herein and in the appended claim v comprises any such arrangement.

Upon the glass plate 55 of the extinction cell I0 is placed the sample lof latex 60 for testing.A If desired, the plate 55 may be of colored glass, thus acting as a color filter in addition to being the base plate for the extinction cell 50. Of

- t course, a color filter may be introduced into any other suitable place in the system, such as interposed between the extinction cell 50 and the objective end of the microscope 8. Also, instead of a single-color filter, an adjustable color screen adapted to selectively interpose desired colors across the field of examination may be used.

The top part of Threaded on the of the light-proof housing l is closed and the source of light 20 sighted through the microscope 3, disclosing to the eye the. image of the filament within av bright spot of less intensity than the image of the filament. The table is 5 moved slowly by the operator through manipulation of the knurled head 3l ofthe micrometer screw in order to interpone the extinction cell 60 between the source of light 23 and the miseroscope 3 .such that the nlament is shown in its maximum brightness as viewed through the extinction cell. As the micrometer Ascrew is moved clockwise or counter-clockwise from the point of maximum brightness of the filament, there is interposed in the line of vision a gradu- 15 ally increasing opacity, which causes the image of the nlament to gradually fade away and ultinnately disappear against a background of diffuse illumination of the microscope field.

This disappearance occurs at .points equally 20 distant from the center of maximum brightness. on each side of the point of maximum brightness. Referring to Figs. 5 and 6, the depth D of the optical path through the latex within the cell is varied -byrot-ating the knurled head 30 on 25 the micrometer screw 3l and thus shifting the cell sideways until extinction of the light filament in the microscope field is obtained at A' and also at B on both sides of the clear central spot 'O (the point of maximum brightness of the 30 image of the filament).

I'he distance W between the points of extinction A and B as indicated by the scales 40 and 3B is dependent upon the amount of opacity in the latex film in the extinction cell, the film 35 regularly increasing in thickness from the center of the cell. The greater the opacity of the latex, the less will be the distance which the cell must be moved from the point ofimaximumbrightness of the filament to the points of extinction there- 40 of. The resulting scale reading is then compared with a table of predetermined and established standard readings computed by the vtrial coagulation or other method. so that the amount of turbidity possessed by the fluid is directly ob- 45 tained by such reading and reference comparison, the standard readings being obtained by a series of examinations of latices of varying accurately known quantities of rubber.

Although the rubbercontent of any aqueous 50 isuspension of rubber may be measured by this instrument, for the sake' of convenience only tests of samples of ammonia-preserved plantation latices, Revertex, a commercial latex concentrated by evaporation to approximately 80% .rubber content, and Jatex, a commercial latex concentrated by centrifuging to approximately 60 percent rubber lcontent, will be illustrated. In the'data hereinafter presented, distilled water is used as the diluent except where otherwise G0 noted.

In order to further illustrate the invention, Figs. '7 to` 13 showing numerous curves plotted either as extinction curves or calibration curves are provided. In Fig. 'I extinction curves for G0 Revertex using the spherical convex lens and also the biprism in the extinction cell are shown. As seen from a perusal of the succeedingA figures, these curves are characteristic of the curves for other samples of latex.

Referring to the curves in Fig'. 7, the extreme turbidity of latex is noted bythe fact that latex films of 20 tovff l%l rubber content thicker than 0.05 mm. completely obscure the incandescent nlament. Ordinary plantation latex generally Cai has a rubber content of 30 to 35%. This table further shows that extinction cells containing either a lens or a special biprism give closely comparable results. However, because of its simplicity and convenience in practice together with its easy procurability, the lens type of cell is probably the more convenient to use.

The straight line portion of the curve of Fig. 8, showing the relation between extinction depths and reciprocales of rubber content, indicates that the turbidity of water diluted latex obeys the concentration-turbidity law when the rubber content does not exceed 15%. Fig. 9 shows the change in the character of the extinction curves when latex serum or glycerin is used as the diluent for latex instead of water. The use of latex serum-as a diluent does not eliminate the minimum in the extinction curves but gives an extinction curve very similar to that obtained with water as a diluent.

Fig. shows the calibration curves for two samples each of plantation latex,`Revertex and Jatex. A-The two samples of each of these types of latex give practically identical calibration curves. Therefore the calibration curves for these six samples are shown as one curve for each type of latex. The observed differences in the three calibration curves are probably due to differences in the effective mean particle size in these various types of latex.

Since the turbidity of latex depends upon the particle size 'and the color of the transmitted light, it is possible to minimize this effect of particle size by'using color filters as disclosed above. In Fig. 1l are shown calibration curves for Revertexobtained with and without color filters, none of which transmitted mono chromatic light. Of these, the green filter is recommended for use in the determination of the rubber content of latex since it is morepleasing to the observer, yields a more sensitive balance, and decreases the spread between the calibration curves for different latices.v In Figs. 12 and 13 are calibration curves for Revertex and plantation latex obtained without and with the green filter, respectively. As is readily apparent, the use of a green lter brings about a considerable decrease in the spread of the two calibration curves.

In making tests with ordinary plantation latex, it is desirable to dilute the latex with water or some other suitable diluent because of the fact that its rubber content falls in the region of the minimum of the extinction curve where two values of the rubber content correspond to a. single value of the extinction depth (see Fig. 7) This results in an ambiguity in coverting values of extinction depth into rubber content if this region of the extinction curve should be used as a calibration curve for the instrument. To avoid this ambiguity, a known dilution of the unknown latex, such as a one-tenth dilution of plantation latex is used. It is then possible to use curves -plotted between extinction widths in micrometerscale divisions and per cent rubber content up to 10 to 14 per cent. as calibration curves for the instrument.

'I'he advantages of using a filter are readily seen by referring to Figures 12 and 13. For example, an unknown sample of latex is diluted one part in ten with water and extinction widths of 24 and 22 scale divisions are observed, respectively, without and with the green lter. Without the filter, the calibration curves with a simple multiplication of 10 would indicate a rubber content of 33 or 39%, depending upon the choice from the two calibration curves in Figure l2. With the filter, the two computed values would be 34 to 35%, as seen in Figure 13.

Thus, the microturbidimeter with a green light filter permits the determination of the rubber content of latex with a probable precision of less than 1% rubber content in 35% latex. The time required for this determination is less than five minutes as compared with two or more hours for the long method involving coagulation, creping, and drying, or a probable half hour for the vless-accurate short method of coagulation and creping without drying.

Another use for this microturbidimeter is in the measuring of particle size of pigments such as zinc oxide, iron oxide and carbon black. One

method which we have found to be quite accurate and highly eilicient is -that based on the turbidity of xylene-rubber-pigment cements, prepared by milling the pigment into the rubber and swelling the mixture in xylene. It is of course to be understood that this microturbidmeter may be used also for measuring the particle size of pigments in aqueous or other media.

In general, the method of determining the particle size involves calibration of the instrument using a direct method of particle-size measurement. With some pigments, such as zinc oxide, an extinction curve with a minimum is obtained when the extinction depths for suspensions of the same concentration are plotted against the particle size. In such a case, the possible ambiguity as to which side of the minimum the pigment belongs may be overcome by taking readings with different color filters, e. g., with a blue filter and with a red lter or with an adjustable color screen. The ratios of the readings taken with these two different filters are not the same on the two sides of the minimum.

From the above considerations it will be seen that a high precision instrument is provided with which turbidity determinations of a fluid may be quickly made. It will be understood that the details of the instrument as herein specifically described and illustrated may be varied without departing from the inventive concept; also, that it' is desired to embrace within the scope of this invention such modifications and changes as may be necessary to adapt it to varying conditions and uses. It is intended that the patent shall cover by suitable expression in the appended claim whatever features of patentable novelty'residein the invention.

What is claimed is: The method of determining the rubber content 'of a dispersion of rubber-like material which comprises observing through a green lter an incandescent filamentv by looking through an extinction cell containing the dispersion, moving the extinction cell in a light proof housing across the eld of vision from the extinction point on one side of the point of maximum brightness to the extinction point on the other side thereof by vmeans which` indicate the amount of movement and observing the indicated amount 'of move- 70 ment.

JOHN S. WARD. SAMUEL D. GEHMAN. 

